docname=rect:Rectangles
<p>
Draw lines along the grid edges to divide the grid into rectangles,
so that each rectangle contains exactly one numbered square and its
area is equal to the number written in that square.
<p>
Click and drag from one grid corner to another, or from one square
centre to another, to draw a rectangle. You can also drag along a
grid line to just draw a line at a time, or just click on a single
grid edge to draw or erase it.
